This page contains a Flash digital edition of a book.
July, 2017


www.us-tech.com


Page 59


The Benefits and Risks of Copper Pillar Bumped Flip Chips: Part 2


By Craig Hillman Ph.D., CEO, DfR Solutions C


opper has inherently higher electrical and thermal conduc- tivity than SnPb or SnAg/


SnAgCu solders, by a factor of about 25. Higher conductivities reduce cur- rent density and temperature within the interconnect, which are the pri- mary drivers for matter flow and elec- tromigration (EM) failures. Thus, for the same dimensions, copper pillar joints should significantly outperform conventional solder bump intercon- nects for electromigration reliability. Finite element simulations and experiments appear to support this


interconnect reliability, which could be of particular concern as substrates get thinner. Improved thermal cycle per-


formance might be expected since copper pillar technology can offer a larger standoff for a given pitch, resulting in more compliance within the interconnect. However, copper has a higher modulus than the tin- based solders used in flip chip con-


struction, such as SAC305, 63Sn- 37Pb and 90Pb-10Sn, by up to a fac- tor of 3. This higher elastic modulus could create a stiffer, less-compliant structure, leading to increased stress in the system after package assembly or during thermal cycling. Finite element studies per-


formed to date are conflicting regard- ing assembly or thermal cycling stresses and strain energy densities


in copper pillar joints compared to conventional solder bumps. Some finite element analyses indicate that stresses are lower in copper pillar joints compared to conventional sol- der joints, while others show worse performance for copper pillars. Another area where finite ele-


ment investigations give mixed pre- dictions concerns the height of the


Continued on page 62


Image showing growth of intermetallics within the solder portion of a copper pillar joint.


expectation. For example, as much as a three-fold increase in EM lifetime has been demonstrated experimen- tally. Both finite element analysis and experimental analysis show this benefit over conventional solder bump technology. Despite such studies, the full


impacts of fine-pitch copper pillar structures on EM reliability have yet to be fully explored. The switch from solder bumps in today’s flip chips to copper pillar will not result in joints of similar dimensions. Fine-pitch sol- der bump joints range from 80 to 100 µm in diameter, while those for cop- per pillar are expected to be from 25 to 60 µm. The cross-sectional area of the critical joint regions where the current is most crowded will drive EM failure, and this area varies with the square of the joint diameter. Therefore, the decreased joint


dimensions are significant. Such reductions are driven by desires to reduce the stress on low-k dielectric layers by reducing the ratio of the bump or pillar diameter to the UBM diameter and the drive to reduce the pitch of first-level interconnect. Also, several studies have shown that cur- rent crowding, which drives EM, is highest at the entrance of the metal trace to the bump. Since these details are impacted by the joint design, EM life, in turn, may vary from design to design. The use of bump on trace (BOT) structures could also introduce areas of elevated current density.


Thermal Cycle Reliability Surprisingly, the thermal


fatigue performance of copper pillar packages has received limited exper- imental investigation, despite a rela- tively large number of finite element studies. Testing of the thermal fatigue performance of packages mounted to circuit boards has not been done in order to test package-to- board interactions on first-level


CondensoX-Series Condensation Soldering


For more information visit or call + 1 770 442 8913


 A patented principle with advantages


 › Voids ratios belowpossible  › Up to  less energy › Prepared for 


 Vacuum


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92